Air Trench in Packages Incorporating Hybrid Bonding

ABSTRACT

A package component includes a surface dielectric layer including a planar top surface, a metal pad in the surface dielectric layer and including a second planar top surface level with the planar top surface, and an air trench on a side of the metal pad. The sidewall of the metal pad is exposed to the air trench.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/042,268, entitled “Air Trench in Packages Incorporation HybridBonding,” filed Feb. 12, 2016, which application is a divisional of U.S.patent application Ser. No. 13/895,122, entitled “Air Trench in PackagesIncorporation Hybrid Bonding,” filed May 15, 2013, now U.S. Pat. No.9,443,796, issued Sep. 13, 2016, which application claims the benefit ofU.S. Provisional Application No. 61/793,355, filed Mar. 15, 2013, andentitled “Air Trench in Packages Incorporating Hybrid Bonding,” whichapplications are hereby incorporated herein by reference.

BACKGROUND

In wafer-to-wafer bonding technology, various methods have beendeveloped to bond two package components (such as wafers) together. Theavailable bonding methods include fusion bonding, eutectic bonding,direct metal bonding, hybrid bonding, and the like. In the fusionbonding, an oxide surface of a wafer is bonded to an oxide surface or asilicon surface of another wafer. In the eutectic bonding, two eutecticmaterials are placed together, and are applied with a high pressure anda high temperature. The eutectic materials are hence melted. When themelted eutectic materials are solidified, the wafers are bondedtogether. In the direct metal-to-metal bonding, two metal pads arepressed against each other at an elevated temperature, and theinter-diffusion of the metal pads causes the bonding of the metal pads.In the hybrid bonding, the metal pads of two wafers are bonded to eachother through direct metal-to-metal bonding, and an oxide surface of oneof the two wafers is bonded to an oxide surface or a silicon surface ofthe other wafer.

The previously developed bonding methods have their advantageousfeatures and disadvantageous features. For example, the fusion bondingrequires low forces, and may be performed at room temperature. However,since there is no electrical connection between the bonded wafers, extraelectrical connections need to be made to interconnect the bondedwafers. The Eutectic bonding does not require high-quality surfaces, andhence the pre-requirement for a successful bonding is loosened. Theaccuracy of the eutectic bonding, however, is low, and there may bemetal-squeeze problem during the eutectic bonding due to the melting ofthe bonding metals. The direct metal-to-metal bonding has a highaccuracy. The throughput, however, is very low. In the hybrid bonding,on the other hand, the metal pads have higher Coefficients of ThermalExpansion (CTEs) than the dielectric layers at the surfaces of thebonded wafers. This results in problems for the bonding of the surfacedielectric layers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 10 are cross-sectional views of intermediate stages inthe bonding of two package components in accordance with some exemplaryembodiments; and

FIG. 11 illustrates a top view of the bonded metal pads, bonded surfacedielectric layers, and air trenches in accordance with some embodiments.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A method for bonding package components through hybrid bonding isprovided in accordance with various exemplary embodiments. Theintermediate stages of the bonding process are illustrated. Thevariations of the bonding process in accordance with the embodiments arediscussed. Throughout the various views and illustrative embodiments,like reference numbers are used to designate like elements.

FIGS. 1 through 10 illustrate cross-sectional views of intermediatestages in a bonding process in accordance with some embodiments of thepresent disclosure. Referring to FIG. 1, package component 100 isillustrated. Package component 100 may comprise a device wafer, apackaged wafer, an interposer wafer, or the like. In the embodimentspackage component 100 comprises a device wafer, package component 100may include semiconductor substrate 102, which may be, for example, asilicon substrate, although other semiconductor substrates such assilicon carbon substrates, silicon germanium substrates, III-V compoundsemiconductor substrate are also usable. Active devices 104 may beformed on a surface of substrate 102, and may include, for example,transistors. Metal lines and vias 106 are formed in dielectric layers108, which may include Inter-Layer Dielectric (ILD), Inter-MetalDielectric (IMD) layers, passivation layers, and the like. The ILD layerand the IMD layers may be low-k dielectric layers in some embodiments,which have dielectric constants (k values) smaller than about 3.5,smaller than about 3.0, or smaller than about 2.5. Dielectric layers 108may also comprise non-low-k dielectric materials having dielectricconstants (k values) greater than 3.9. Metal lines and vias 106 maycomprise copper, aluminum, nickel, tungsten, or alloys thereof. Metallines and vias 106 interconnect active devices 104, and may electricallyconnect active devices 104 to the overlying metal pads 112.

In alternative embodiments, package component 100 is an interposerwafer, which is free from active devices therein. Package component 100may, or may not, include passive devices (not shown) such as resistors,capacitors, inductors, transformers, and the like in accordance withsome embodiments.

In yet alternative embodiments, package component 100 is a packagesubstrate. In some embodiments, package component 100 is a laminatepackage substrate, wherein conductive traces 106 (which areschematically illustrated) are embedded in laminate dielectric layers108. In alternative embodiments, package components 100 are built-uppackage substrates, which comprise cores (not shown), and conductivetraces (represented by 106) built on opposite sides of the cores. Theconductive traces 106 are interconnected through conductive features inthe cores.

In each of the embodiments wherein package component 100 is a devicewafer, an interposer wafer, a package substrate, or the like, surfacedielectric layer 110 may be formed at the surface of package component100. In some embodiments, surface dielectric layer 110 is an oxidelayer, which may comprise silicon oxide. In alternative embodiments,surface dielectric layer 110 comprises other materials such as SiON,SiN, or the like. Metal pads 112 are formed in surface dielectric layer110, and may be electrically coupled to active devices 104 through metallines and vias 106. Metal pads 112 may also be formed of copper,aluminum, nickel, tungsten, or alloys thereof. The top surface ofsurface dielectric layer 110 and the top surfaces of metal pads 112 aresubstantially level with each other. In the embodiments wherein packagecomponent 100 is a device wafer, surface dielectric layer 110 and metalpads 112, which are used for the subsequent bonding, may be on the frontside (the side with active devices 104) or the backside of substrate102. For example, FIG. 1 illustrates that surface dielectric layer 110and metal pads 112 are on the front side of substrate 102.

FIGS. 2-5 illustrate the photolithography processes for forming airtrenches.

Referring to FIG. 2, photo resist 114 is formed over package component100, and is then patterned. The portions of surface dielectric layer 110that encircle metal pads 112 are exposed through the openings in photoresist 114. The exposed portions of surface dielectric layer 110 mayform rings encircling metal pads 112 in some exemplary embodiments. Insome embodiments, metal pads 112 are also exposed, although metal pads112 may also be covered by the remaining photo resist 114.

Next, as shown in FIG. 3, surface dielectric layer 110 is etched, forexample, using a dry etching method. The etchant gas may be selected toattack surface dielectric layer 110, and not to attack metal pads 112.Hence, air trenches 116 are formed to encircle metal pads 112. Exemplaryair trenches 116 are shown in the top view in FIG. 11. Referring back toFIG. 3, the sidewalls of metal pads 112 are exposed to air trenches 116.In addition, the sidewalls of metal pads 112 and the sidewalls ofsurface dielectric layer 110, which sidewalls are exposed to airtrenches 116, may be substantially vertical. Depth D1 of air trenches116 may be equal to thickness T1 of surface dielectric layer 110 in someembodiments. Alternatively, depth D1 may be greater than or smaller thanthickness T1. Width W1 of air trenches 116 may be between about 0.1 μmand about 2 μm, greater than about 0.5 μm, or between about 0.5 μm andabout 2 μm. It is appreciated, however, that the values recitedthroughout the description are merely examples, and may be changed todifferent values. After the etching step, photo resist 114 is removed.

FIGS. 4 and 5 illustrate the formation of air trench 116 in accordancewith alternative embodiments. These embodiments are similar to theembodiments in FIGS. 2 and 3, except that the outer portions of metalpads 112, rather than portions of surface dielectric layer 110, areetched. Referring to FIG. 4, photo resist 114 is formed and patterned.The center portions of metal pads 112 are protected by portions of photoresist 114, while the outer portions metal pads 112 are exposed.Although surface dielectric layer 110 is shown as entirely covered byphoto resist 114, the openings in photo resist 114 may also extendslightly to expose some portions of surface dielectric layer 110 toallow some process margin.

Next, referring to FIG. 5, metal pads 112 are etched to form airtrenches 116. The etchant gas may be selected to attack metal pads 112,and not to attack surface dielectric layer 110. Air trenches 116 mayform rings encircling the remaining portions of metal pads 112, as shownin the top view in FIG. 11 in some embodiments. As shown in FIG. 5, thesidewalls of the remaining metal pads 112 are exposed to air trenches116. Depth D1 of air trenches 116 may be equal to thickness T2 of metalpads 112 in some embodiments. Alternatively, depth D1 may be greaterthan or smaller than thickness T2. Width W1 of air trenches 116 may bebetween about 0.1 μm and about 2 μm, greater than about 0.5 μm, orbetween about 0.5 μm and about 2 μm. After the etching step, photoresist 114 is removed.

FIG. 6 illustrates package component 200, which is to be bonded topackage component 100. Package component 200 may also be selected from adevice wafer, an interposer wafer, a package substrate, and the like. Inthe illustrated FIG. 2, package component 200 includes substrate 202,active devices 204, dielectric layers 208, metal lines and vias 206 indielectric layers 208, surface dielectric layer 210, and metal pads 212.Package component 200 may have a structure similar to what is describedfor package component 100, and the details are not repeated herein. Thematerials of the features in package component 200 may be foundreferring to the like features in package component 100, with the likefeatures in package component 100 starting with number “1,” whichfeatures correspond to the features in package component 200 and havingreference numerals starting with number “2.”

FIGS. 6 and 7 also illustrate the formation of air trenches 216 inaccordance with various embodiments. The processes are essentially thesame as in FIGS. 2 and 3, and hence some of the details may not berepeated herein. Referring to FIG. 6, photo resist 214 is formed,wherein portions of surface dielectric layer 210 are exposed through theopenings in photo resist 214. Metal pads 212 may also be exposed,although they may be covered in alternative embodiments.

Next, as shown in FIG. 7, surface dielectric layer 210 is etched, forexample, using a dry etching method. The etchant gas may be selected toattack surface dielectric layer 210, and not to attack metal pads 212.Hence, air trenches 216 are formed to encircle metal pads 212. Airtrenches 216 may form rings encircling metal pads 212, as shown in thetop view in FIG. 11. As shown in FIG. 7, the sidewalls of metal pads 112are exposed to air trenches 216. Depth D2 of air trenches 216 may beequal to thickness T3 of surface dielectric layer 210 in someembodiments. Alternatively, depth D2 may be greater than or smaller thanthickness T3. Width W2 of air trenches 116 may be essentially the sameas width W1 (FIG. 3). After the etching step, photo resist 214 isremoved.

FIGS. 8 and 9 illustrate the etching of metal pads 212 to form airtrenches 216 in accordance with alternative embodiments. Theseembodiments are similar to the embodiments in FIGS. 4 and 5. Referringto FIG. 8, photo resist 214 is formed and patterned. The center portionsof metal pads 212 are protected by a portion of photo resist 214, whilethe outer portions are exposed. Although surface dielectric layer 210 isshown as covered by photo resist 214, the openings in photo resist 214may also extend slightly to expose some portions of surface dielectriclayer 210 to allow some process margin.

Next, referring to FIG. 9, metal pads 212 are etched to form airtrenches 216. The etchant gas may be selected to attack metal pads 212,and not to attack surface dielectric layer 210. Air trenches 216 mayform rings encircling the remaining portions of metal pads 212, as shownin the top view in FIG. 11. As shown in FIG. 9, the sidewalls of theremaining metal pads 212 are exposed to air trenches 216. Depth D2 ofair trenches 216 may be equal to thickness T4 of metal pads 212 in someembodiments. Alternatively, depth D2 may be greater than or smaller thanthickness T4. Width W1 of air trenches 216 may be essentially the sameas width W1 (FIG. 5). After the etching step, photo resist 214 isremoved.

Next, as shown in FIG. 10, package components 100 and 200 arepre-bonded. In the pre-bonding, package components 100 and 200 are firstaligned, with bond pads 112 of package component 100 aligned to bondpads 212 of package component 200. After the alignment, packagecomponents 100 and 200 are pressed against each other. During thepre-bonding, a small pressing force may be applied to press packagecomponents 100 and 200 against each other, which pressing force may belower than about 5 Newton per die, for example. The pre-bonding may beperformed at the room temperature (for example, between about 21° C. toabout 25° C.), although higher temperatures may be used. The bondingtime may be shorter than about 1 minute, for example.

After the pre-bonding, surface dielectric layer 110 and 210 are bondedto each other. The bonding strength, however, needs to be improved in asubsequent annealing step. The bonded package components 100 and 200 maybe annealed at a temperature between about 300° C. and about 400° C.,for example. The annealing may be performed for a period of time betweenabout 1 hour and 2 hours. When temperature rises, the OH bond in surfacedielectric layers 110 and 210 break to form strong Si—O—Si bonds, andhence package components 100 and 200 are bonded to each other throughfusion bonds (and through Van Der Waals force). In addition, during theannealing, the copper in metal pads 112 and 212 diffuse to each other,so that metal-to-metal bonds are also formed. Hence, the resulting bondsbetween package components 100 and 200 are hybrid bonds. After thebonding, the bonded package components 100 and 200 are sawed intopackages, with the portions of package components 100 and 200 in thepackages comprising dies.

As shown in FIG. 10, air trenches 116 and 216 may be aligned to eachother, and are interconnected to form continuous air trenches extendingfrom package component 100 into package component 200. Furthermore, theintegrated air trenches 116 and 216 encircle the bonded metal pads 112and 212.

FIG. 11 illustrates a top view showing air trenches 116 and 216 areoverlapped, and metal pads 112 and 212 are stacked. It is appreciatedthat although the term “air trench” is used, trenches 116 and 216 arenot necessarily filled with air. Depending on the environment in whichthe pre-bonding and annealing is performed, trenches 116 and 216 may bevacuumed, or may be filled with other types of gases such as nitrogen.In some embodiments, as illustrated in FIG. 11, air trenches 116 and 216encircle bonded metal pads 112 and 212. In other embodiments, airtrenches 116 and 216 do not extend into regions 18. Hence, air trenches116 and 216 may be formed on the opposite sides of metal pads 112 and212, and do not form rings in these embodiments.

In the embodiments of the present disclosure, due to the formation ofair trenches, free spaces are generated around the bonded metal pads.During the annealing process, the metal pads expand more than thesurface dielectric layers since their Coefficient of Thermal Expansion(CTE) is greater than the CTE of the surface dielectric layers. Withoutthe air trench, the expanded metal pads apply stresses to thesurrounding dielectric layers, and hence cause damage to the fusionbonds of the surrounding dielectric layers. With the air trenches, freespaces are created to allow the metal pads to expand freely withoutapplying stresses on the surrounding dielectric layers, and hence thesurrounding dielectric layers may have good fusion bonds formed.

In accordance with some embodiments, a package component includes asurface dielectric layer including a planar top surface, a metal pad inthe surface dielectric layer and including a second planar top surfacelevel with the planar top surface, and an air trench on a side of themetal pad. The sidewall of the metal pad is exposed to the air trench.

In accordance with other embodiments, a method includes performing aphoto lithography process on a first package component to form an airtrench in a first surface dielectric layer of the first packagecomponent. The air trench encircles a first metal pad in the firstsurface dielectric layer. The method further includes bonding the firstpackage component to a second package component. The first metal pad isbonded to a second metal pad of the second package component. The firstsurface dielectric layer is bonded to a second surface dielectric layerof the second package component.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A package comprising: a first package componentcomprising: a first surface dielectric layer; a first metal pad in thefirst surface dielectric layer; and a first air trench, wherein a firstsidewall of the first metal pad is exposed to the first air trench; anda second package component over and bonded to the first packagecomponent, wherein the second package component comprises: a secondsurface dielectric layer; and a second metal pad in the second surfacedielectric layer, wherein the second package component comprises aportion exposed to the first air trench.
 2. The package of claim 1,wherein the first sidewall of the first metal pad is aligned to a secondsidewall of the second metal pad, and the first air trench extends intothe second package component, with the second sidewall exposed to thefirst air trench.
 3. The package of claim 1, wherein the first airtrench extends to a bottom surface of the first surface dielectriclayer.
 4. The package of claim 1, wherein the first air trench extendsto an intermediate level between a top surface and a bottom surface ofthe first surface dielectric layer.
 5. The package of claim 1, whereinthe first air trench forms a trench ring encircling the first metal pad.6. The package of claim 1 further comprising a second air trench,wherein the first air trench and the second air trench are on oppositesides of the first metal pad, and the first air trench and the secondair trench are disconnected from each other.
 7. The package of claim 1,wherein the first air trench has a width greater than about 0.5 μm.
 8. Apackage comprising: a first package component comprising: a firstsurface dielectric layer; a first metal pad in the first surfacedielectric layer; a first air trench with a portion on a side of thefirst metal pad, wherein a sidewall of the first metal pad is exposed tothe first air trench; and a second package component over and bonded tothe first package component, wherein the second package componentcomprises: a second surface dielectric layer; a second metal pad in thesecond surface dielectric layer, wherein the first metal pad is bondedto the second metal pad; a second air trench with a portion on a side ofthe second metal pad, wherein a sidewall of the second metal pad isexposed to the second air trench.
 9. The package of claim 8, wherein thefirst air trench is joined to the second air trench.
 10. The package ofclaim 8, wherein the first air trench forms a ring encircling the firstmetal pad, and all sidewalls of the first metal pad are exposed to thefirst air trench.
 11. The package of claim 8, wherein the first packagecomponent further comprises a third air trench, wherein the first airtrench and the third air trench are on opposite sides of the first metalpad, and the first air trench and the third air trench are disconnectedfrom each other.
 12. The package of claim 11, wherein in a top view ofthe package, the first air trench has a length greater than a respectivelength of the first metal pad.
 13. The package of claim 8, wherein thefirst air trench extends to a bottom surface of the first surfacedielectric layer.
 14. The package of claim 8, wherein the first airtrench extends to an intermediate level between a top surface and abottom surface of the first surface dielectric layer.
 15. A packagecomprising: a first package component comprising: a first surfacedielectric layer; a first metal pad in the first surface dielectriclayer, wherein a top surface of the first surface dielectric layer and atop surface of the first metal pad are coplanar; a first air trench witha first portion on a first side of the first metal pad, wherein a firstsidewall of the first metal pad is exposed to the first air trench; anda second air trench with a second portion on a second side of the firstmetal pad, wherein a second sidewall of the first metal pad is exposedto the second air trench, and the first sidewall and the second sidewallare opposite sidewalls of the first metal pad.
 16. The package of claim15, wherein the first air trench and the second air trench are discreteair trenches separated from each other.
 17. The package of claim 15,wherein the first air trench has a length greater than a respectivelength of the first sidewall of the first metal pad, and the first airtrench extends beyond an end of the first sidewall.
 18. The package ofclaim 15 further comprising a second package component over and bondedto the first package component, wherein the second package componentcomprises: a second surface dielectric layer; a second metal pad in thesecond surface dielectric layer, wherein the first metal pad is bondedto the second metal pad; and a third air trench with a third portion ona side of the second metal pad, wherein a third sidewall of the secondmetal pad is exposed to the third air trench.
 19. The package of claim18, wherein the first air trench is joined to the third air trench. 20.The package of claim 18, wherein the second package component furthercomprises: a fourth air trench with a fourth portion on a side of thesecond metal pad, wherein a fourth sidewall of the second metal pad isexposed to the fourth air trench, and the third air trench and thefourth air trench are separated from each other, and the fourth airtrench is joined to the second air trench.